==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.1
- Deployed with iRedMail Easy or the downloadable installer? downloadable installer
- Linux/BSD distribution name and version:  UBUNTU 16.04 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  MySQL
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? yes
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Dear forum members,

We have the following setup:
one iredmail server serving xxx.xxx domain
the second iredmail server serves yyy.yyy domain

There is an alias for support@yyy.yyy which has an entry support@xxx.xxx and some others.

When support@xxx.xxx sends an email to support@yyy.yyy all the alias entries gets the mail, except the return back support@xxx.xxx, with error:

"
This is the mail system at host yyy.yyy.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

                   The mail system

<support@xxx.xxx>: host xxx.xxx[1.1.1.1] said: 554 5.7.1
    <supportl@xxx.xxx>: Recipient address rejected: SMTP AUTH is required for
    users under this sender domain (in reply to RCPT TO command)

"

Can you please advise how can I get this problem solved?

Thanks,
Balays